1. Field of the Invention
The invention relates to a device for write precompensation of data signals to be recorded on a magnetic medium, which device has an input for receiving input data signals and an output for supplying output data signals to be recorded, at least some of the signal transitions in these output data signals being delayed with respect to the corresponding signal transitions in the input data signal.
2. Description of Related Art
In the high-density recording of data on magnetic media non-linear bit shifts are produced when certain data patterns occur. These bit shifts are mainly caused by demagnetization effects component separation mechanism, i.e. due to the fact that the magnetic medium "opposes" the recording of a sign reversal in the bit stream, particularly when a plurality of bits of the same sign precede a bit of an opposite sign. This effect manifests itself in that a sign reversal is recorded too late or too early, as a result of which bits are recorded which are too short or too long, respectively. Such an inaccuracy in the length of a recorded bit is highly undesirable, particularly in the case of high recording and read densities, because the reliability of the bit detection in the read signal is then reduced significantly.
It is known that said effect can be precluded at least partly by the use of write precompensation, which means that a sign reversal to be recorded is either advanced or delayed depending on the value of a number of preceding bits. It is also known to consider for this purpose merely the last preceding bit, referred to as first-order precompensation, or the last two preceding bits, referred to as second-order precompensation. Recently, it has also been proposed to apply a third-order precompensation.
U.S. Pat. No. 4,607,295 describes a device which comprises a transversal filter and by means of which it is decided, depending on the sign of two preceding bits, not to delay a bit to be recorded or to record this bit with a first or a second fixed delay.
U.S. Pat. No. 4,964,107 describes a device by means of which it is decided, depending on the sign of two preceding bits, whether or not to delay the bit to be recorded. Here, the selected delay also depends on the location where the data is recorded on a magnetic disc, for which purpose a choice can be made from eight delay values.
Finally, U.S. Pat. No. 5,325,241 describes a write precompensation device in which the output voltage of an integrator, which is formed by a capacitance, is compared with three voltage values. Depending on which of the three voltage values is equal to the integrator voltage a bit to be recorded is not delayed, or delayed by a first time, or delayed by a second time.
A drawback of these known write precompensation devices is that they only permit a very limited number of delays to be chosen for the compensation of non-linear bit shifts but in this case allowance is made for only a very limited number of preceding bits.
Another effect which occurs in the high-density recording of data on magnetic media, besides said non-linear bit shift, is known as "partial erasure". Partial erasure can occur when one bit cell of minimum width is recorded, i.e. a bit cell bounded at both sides by a sign reversal, also called a transition, and can be explained as resulting from the partial overlapping of the boundaries at either side of the bit cell owing to inaccuracies of these boundaries. As a result of partial erasure a recorded bit cannot be detected properly or even not at all due to the reduced magnetization.
In the article "Characterization of the Read/Write Process for Magnetic Recording" by Dean Palmer, John Hong, David Stanek and Roger Wood in IEEE Transactions on Magnetics, vol. 31, no. 2, March 1995, pages 1071-1076, it is proposed to mitigate the effect of partial erasure by moving the two transitions, which bound a single bit cell of minimum width, away from each other by a small amount. By means of the method proposed in said article it is, in fact, ascertained whether there are transitions between the consecutive bits a.sub.k-2, a.sub.k-1, a.sub.k and a.sub.k+1 and, depending on the detected pattern of transitions, the shift of the transitions bounding the bit cell a.sub.k is determined. The practical implementation of this method is fairly intricate.
A third effect which introduces non-linear bit shifts has its root in the limited bandwidth of the recording channel. A transition produces a comparatively slow variation of the magnetic field in the write head by means of which the signals are recorded on the magnetic medium. A reversal of the magnetization of the medium does not occur and recording of the transition is not effected until the magnetic field strength exceeds a given threshold value. At high data rates the overall response to a signal transition extends over many bit intervals and thus influences the recording of the transitions of many subsequent bits. In practice, it has been found that this effect can result in the transitions being shifted relative to one another by an amount of 20% or more of one bit interval.